The present invention relates to yarn stopping devices for premeasuring weft feeders of air-jet looms.
Conventional weft feeders are devices designed to accumulate a reserve of yarn in the form of turns wound on a fixed drum and to feed the loom by unwinding the accumulated turns in an amount equal to the length L of the yarn required by the loom at each beat; such length is equal to the transverse dimension or width of the fabric being formed.
In the specific case of air-jet looms, the weft feeder is also designed to premeasure the length L; this function is performed by counting, by means of a photoelectric sensor associated with a microprocessor, the unwound turns of yarn, with EQU L=n.pi.D
where n is the number of unwound turns and D is the diameter of the drum of the weft feeder.
The unwinding of the yarn is controlled by a stop device, typically of the electromagnetic type, which by means of a stop finger which can move radially with respect to the drum engages the yarn, blocking its unwinding, when the n-th turn is reached.
More specifically, when the last-but-one turn n-1 is counted, the photoelectric sensor and the cooperating microprocessor activate the power supply of the stop device, whose intervention is completed after the unwinding of the subsequent last turn (n).
As can be appreciated, the intervention time t available to the stop device is very short. It is typically between 10 and 20 ms (milliseconds) in view of the speed of modern air-jet looms, which are capable of inserting approximately 1,500 meters of weft per minute.
These demanding operating conditions have posed, and still pose, considerable problems in the provision of electromagnetic stop devices, which must have extremely high intervention speeds and must have moving parts which have a very small inertia but, at the same time, a high magnetomotive force and an acceptable life, for example equal to four or five years of operation, and therefore the ability to perform approximately 10.sup.9 operating cycles without being damaged.